How are vertex normals being calculated regarding object origin?
To workaround this extrude faces of that surface to form a volume; faces will be extruded along their normals which have roughly the same orientation as vertex normals in this case (except for the corners where vector differs because of one additional orientation):



Then vectors are suitable for such tasks like scaling along normals as they roughly got desired orientation.
This is more a workaround as extrusion might not be convenient, but it's always possible to hide faces around mentioned and work only with those needed.